1. Field of the Invention
The present invention relates to 4-stroke engines capable of working according to the self-ignition process and wherein the exhaust gas is recycled to the intake.
2. Description of the Prior Art
Self-ignition consists in using residual burned gases for initiating combustion in the combustion chamber itself. It is well-known that self-ignition is controlled via the amount of burned gases recycled and the combination thereof with the fuel mixture. The hot burned gases initiate combustion of the fuel mixture thanks to a specific combination of temperature and of active species (free radicals whose concentration reaches a critical value when self-ignition takes place).
It is well-known that combustion in this type of engines has advantages as regards emissions: low hydrocarbon emissions are obtained for two-stroke engines (40 to 80% decrease in relation to a conventional engine) and nitrogen oxides emissions can be decreased by up to 50%. For four-stroke engines, nitrogen oxides emissions can even be lower (close to ambient levels under certain conditions).
Furthermore, a remarkable combustion regularity is obtained with self-ignition running.
In two-stroke engines, the presence of residual gases (gases burned during the previous cycle or cycles) in the combustion chamber is inherent in the operation: in fact, when the load of the engine decreases, the amount of fresh gases decreases and is naturally replaced by an amount of residual gases. The two-stroke engine thus runs with an internal recirculation of burned gases, notably at partial load.
However, the presence of these burned gases in the combustion chamber is not sufficient to obtain the desired self-ignition running. Research work shows that the mixing of the residual gases and of the fresh gases has to be controlled and limited.
In adapting self-ignition technology for two-stroke engines to four-stroke engines, it is necessary to either increase the compression ratio considerably, or to heat up the engine notably, or to combine both phenomena.
French patent 2,738,594 describes a process which partially solves these problems by means of suitable additives in the fuel.
International application WO 93/16,276 discloses a variable distribution associated with a non-return system at the intake providing a decrease in pumping losses at partial load.
French patent application 97/02,822 filed by the assignee describes a self-ignition control in a four-stroke engine. More precisely, this document recommends, at partial load, to minimize mixing of the fresh gases with the burned gases confined in the combustion chamber (internal EGR) by delayingclosing of the exhaust as much as possible.